Organic electroluminescent devices are attracting attention as next-generation display devices, and recently, there are increasing intensively studies for development of various organic materials for use in such light-emitting devices. In particular, as the light-emitting materials, phosphorescent materials that use emission from excited triplet state are attracting attention.
When emission from excited singlet state is used, the probability of generation of the light-emitting excited species is only 25% since the singlet excitons and triplet excitons are generated at a ratio of 1:3 and the emission extraction efficiency is approximately 20%, and thus the external extraction quantum efficiency is at most 5%. On the other hand, if emission from the excited triplet state is also used, the maximum internal quantum efficiency is 100%, and the emission efficiency becomes in principle 4 times larger than that in the case of emission from the excited singlet state. For that reason, intensive studies for development of organic electroluminescent devices using a phosphorescent material are under progress. In particular, as the phosphorescent materials, orthometalated iridium complexes, including as a typical example tris(2-phenylpyridine)iridium complexes, are compounds which particularly attracting attention (Patent Document 1). However, this type phosphorescent material shows only green emission, and there is a need for development of another novel phosphorescent material.
Patent Document 1: WO 00/70655 Pamphlet